1. Field of the Invention
The present invention relates to a humidifier unit for a refrigerated display cabinet, and more particularly to a humidifier unit for humidifying cooled air in a refrigerated display cabinet to preserve fresh produce stored in the display cabinet for a long period of time.
2. Description of the Prior Art
U.S. Pat. No. 4,179,900 discloses an apparatus for preserving fresh produce displayed on shelves in a refrigerator for an extended period of time by supplying mist from a plurality of nozzles positioned above the shelves to the produce articles being displayed. However, the disclosed preserving apparatus have had the following disadvantages:
(1) A uniform supply of mist over the produce articles on the shelves requires a certain number of fixed nozzles which need a complicated piping that is expensive.
(2) The nozzles should preferably be located in an upper central position over the shelves. The inlet pipes thus extend from behind the refrigerator cabinet to the nozzles in the refrigerator cabinet, and such a piping system is unsightly to view. The piping system reduces the storage space in the refrigerator cabinet. If large produce articles are placed on the shelf directly below the nozzles, then only such large articles are humidified, but other articles are supplied with little or no mist.
(3) Where the inlet pipes extending to the nozzles in the refrigerator cabinet are long, there is a tendency for water in the pipes to get frozen when the mist is not supplied for a long interval of time. If this happens, then the apparatus cannot be actuated until after the ice in the pipes is thawed.
(4) Since the mist is supplied from the nozzles, mist particles are relatively large, and cannot be spread uniformly over a wide area.
There has been proposed to use an ultrasonic humidifier to solve the above drawback (4). The ultrasonic humidifier which preserves fresh produce for a long period of time by humidifying the interior of a refrigerator is operated through the year irrespectively of the seasons. To increase the durability in operation, the ultrasonic humidifier has an atomizing water tank made of stainless steel sheet. The ultrasonic humidifier has an ultrasonic vibrator energized by a transistor including an outer surrounding collector held in direct contact with a cooling plate attached to an outer side wall surface of the atomizing water tank for radiating heat from the transistor and preventing noise. Therefore, the wall of the water tank and the electrode surface of the ultrasonic vibrator which is in contact with the water are electrically connected to each other. Since the natural potential of the stainless steel tank wall is lower than that of an Ni-plated layer on the electrode surface, the following chemical reaction takes place: EQU Ni.fwdarw.Ni.sup.++ +2e.sup.-
so that Ni is dissolved due to galvanic corrosion. The ultrasonic vibrator is then deteriorated at such a rate that it will become inoperative after about 1,000 hours of use. Therefore, the ultrasonic vibrator has to be replaced at frequency intervals regardless of the fact that atomizing water tank is made of stainless steel for increased durability.